The present invention relates to screws adapted for use in snugly-fitted tightening pairs. The invention further refers to wrenches or spanners for operating said screws. In the technology of snugly tightening pairs, screws are made in which the engagement surface of same with the corresponding wrench, consisting either of the outer surface of the screw head, or the side surface of an axial socket formed in the head itself, can take several different configurations.
The most used configurations for screw heads are those in the form of a square, a hexagon, a triangle, a double square, a double hexagon or a double triangle and those involving a socket of same shape. All these geometrical figures however consist of regular polygons so as to enable them to be operated in any angular position. In all the above typologies however, the reaction surfaces between the wrench and screw during the tightening action do not belong to radial planes passing through the rotation axis of the screw itself. Since the tightening force transmitted from the wrench to the screw, due to the application of a given driving torque or torque wrench setting, is directed perpendicularly to the reaction surface at the contact point, whereas the useful tightening force is directed tangentially of a circumference having as its center the screw axis and passing through the contact point, i.e. directed perpendicularly to said radial planes, it results therefrom that the more the reaction surfaces diverge from the radial planes, the lower the useful tightening force one can actually have at one's disposal as compared with the transmitted tightening force.
In particular, the efficiency in tightening of hexagonal-head or hexagonal-socket-head screws is very low (in the order of 50%).
In order to obviate a weak efficiency in these fastening pairs, cross-slotted screws have become increasingly more used, or in any case screws are used which have a head provided with a plurality (often six) of radial lobes or a lobate socket, so that the reaction surfaces lie at least partly in said substantially radial planes.
Screws provided with lobate heads, when tightening or unscrewing is to be carried out, need a wrench having a mating shape in order to achieve a high efficiency in terms of useful tightening force.
In the absence of such a wrench, operation of said screws would be very difficult. For this reason, the socket faces between the lobes or the external faces of the lobes of a lobate head respectively are such shaped that their section with a plane perpendicular to the screw axis belongs to one theoretical regular polygon alone; thus the screws, in case of emergency, can be operated by a traditional wrench, a hexagonal one for example in the case of a six-lobe screw.
In addition, due to the backlash existing between the wrench and screw, when a lobate screw is operated with a polygonal wrench, there is the risk that the lobate profile of said screw may be deformed at the lobe root due to crushing of the corners, so that subsequent insertions of the mating wrench are inhibited.
It is a general object of the present invention to obviate the above drawbacks by providing a screw having a lobate head or a lobate-socket head susceptible of being operated by a wrench of a mating profile, so as to achieve a high efficiency for the tightening pair, and also adapted to be operated by a traditional polygonal wrench, with a higher efficiency than obtainable with a similar coupling made with a lobate screw of the known art, without involving any risk that the corners of the reaction surface thereof will be irreparably deformed.